One of the most significant papers ever published in the annals of science appeared recently; it deals, for the first time ever, with one of the biggest scientific questions ever faced by the scientific community, and uses cutting-edge technology and awesome powers of deductive reasoning and logic to reach shocking, paradigm-shifting conclusions. I refer, of course, to Don Henderson and Darren Naish’s Journal of Theoretical Biology article ‘Predicting the buoyancy, equilibrium and potential swimming ability of giraffes by computational analysis’. I’m sure you’ve already read it and have joined the Facebook group, but for the few of you that aren’t familiar with this sure-to-win-awards marvel of the peer-review system, here’s the low-down…The background to this research

Everybody loves giraffes, and god knows they’ve been covered on Tet Zoo enough times (see the links below). And something that’s been mentioned many times is the alleged inability of giraffes to swim, or even to float. There are several specific comments on this in the literature (e.g., Shortridge 1934, Goodwin 1954, MacClintock 1973, Wood 1982); Crandall (1964) mentioned a case where a captive giraffe escaped from a carrying crate, fell off the end of a jetty, and immediately sank in the Hudson River (incidentally, dead giraffes have apparently been in the Hudson more than once: it is alleged that the river has, on occasion, been used as a dump for the corpses of zoo animals).

Why should giraffes be unable to swim? I’m extremely sceptical of such claims, given that other animals sometimes said to be unable to swim – like giant tortoises, pigs, rhinos and camels – actually swim just fine or even very well. Nevertheless, giraffes are such a strange shape that one can just about believe that their swimming and/or floating behaviour differs from that of other quadrupedal mammals.

After seeing the Focus magazine piece (this was in 2008, I think), I came up with an idea. I knew that my august colleague Donald Henderson of the Royal Tyrrell Museum of Palaeontology – well known for his work on what you might call mathematic palaeontology – had produced a digital model of a giraffe for a previous project (Henderson 1999), and had been testing the buoyancy of some of his models in ‘digital water’ (Henderson 2003a, b, 2006). Could, I wonder, Don take his digital giraffe and drop it into digital water, and thereby test the hypothesis that giraffes cannot float, or cannot swim? This, my friends, is how papers are born…

Modelling a digital giraffe

To start with, it’s clear that we wouldn’t actually be able to fully test the swimming abilities of giraffes (given that we couldn’t accurately replicate or estimate all the details of aquatic movement in a giraffe), but we could test the floating abilities, and hence indirectly test the potential swimming abilities.

Giraffes are complicated objects, and modelling them digitally is fraught with difficulty. A 3D giraffe model was generated via the digital slicing method described by Henderson (1999) (our model isn’t definitely meant to depict any of the Giraffa taxa in particular; yes, we’re well aware of the suggestion that the several ‘subspecies’ conventionally included in G. camelopardalis might warrant species status). Several things were done to make the model more like a real giraffe: ears, ossicones and the fleshy part of the tail were created, and a synthesized reticulated pattern was created because it made the model look so much better. Don has actually pioneered a very interesting technique for generating regularly spaced polygons and/or blotches – it has some really interesting implications that I can’t discuss here (for more information, see Appendix A in Henderson & Naish (2010)).

Two details of a digital model are particularly important when testing simulated buoyancy: density and lung volume.

Giraffe limb bones are slightly thicker than those of other artiodactyls (van Schalwyk et al. 2004), so their density was set as being somewhat higher than the rest of the body (1050 g/l vs 1000 g/l). The giraffe’s most notable feature – its neck – was set at 850 g/l, as is typical for hoofed mammals. As we’ll see, the high density of the distal parts of the limbs and low density of the neck and head seem to have implications for the floating and swimming behaviour of giraffes.

The lungs of giraffes are peculiar in size and shape (as you’ll know if you watched Inside Nature’s Giants), and exactly how large they are has long been a point of controversy: estimates of their volume in an ‘average’ giraffe (of 1.1 tons) have ranged from a low of 10 litres (Patterson et al. 1957) to a high of 47 litres (Robin et al. 1960). By scaling up from a horse to a big, male, 1.6 ton giraffe, an enormous projected lung volume of 141 litres was obtained (lung volume is about 6 litres in humans). Projections of this size have long been thought to be erroneous, however (Stahl 1967). A probable lung size about eight times that of a human is generally thought about right… in which case about 48 litres is correct for a 1.1 ton giraffe; this scales to 74 litres for a 1.6 ton giraffe. Obviously, there’s much uncertainty here and more data is needed.

Don also modelled a horse Equus caballus [shown here] as a control for the experiment.

Can a giraffe float?

Given that the horse model seems to accurately predict the real-life buoyancy of floating and swimming horses (Henderson & Naish 2010), we’re fairly confident that the giraffe model mimics the real thing. By rising the simulated water level around the giraffe model [as shown in the figure below], it was found that an adult giraffe would start to float at a water depth of about 2.8 m. It seems that the hindlimbs would leave the substrate before the forelimbs, raising the possibility that giraffes in deep water might be able to pole themselves along with their forelimbs alone.

But if the water becomes deeper still, what happens when the giraffe lifts off the substrate and begins to float? In a simulated floating posture, the hips are higher than the shoulders and the heavy forelimbs and short body mean that the foreparts are angled downwards relative to the horizontal. In turn, this means that the neck is rotated downwards and has to be held near-horizontally and just at, or under, the water surface: so far as we can tell, it doesn’t seem possible for giraffes to swim with the neck held erect out of the water. The horizontal neck posture then means that the head has to be held at a very awkward, upward-angled posture (assuming, of course, that the animal wants to keep its eyes and nostrils above the surface).

It’s well known that the giraffe head – particularly that of old males – is dense-boned and fairly heavy, but the cavities in the head and the long trachea and oesophagus make the head and neck rather less dense than the rest of the animal (850 g/l vs 1000 g/l or more). The neck and head account for about 10.8% of the animal’s total mass, but it seems that their low density actually prevents the animal’s anterior part from being even further down in the water (Henderson & Naish 2010). The overall density of the giraffe is higher than that of the horse, making the animal closer to being negatively buoyant and also making it sit lower in the water than the horse model.

In conclusion, it seems that giraffes can float: there’s no reason to assume that they might ‘sink like stones’, nor was there any indication from the model that it was particularly unstable and prone to capsizing. But the model’s posture in the water is low and hardly ideal, and looks downright uncomfortable. What might this mean for swimming ability?

Bad at swimming, or really bad at swimming?

The high mean density of the giraffe and the peculiar and difficult horizontal-necked posture the model adopts in water suggest that giraffes would not perform well in water. A large amount of drag and rotational inertia also afflict the model: in fact, it suffered from 13.5% more frictional drag than the horse model (Henderson & Naish 2010). We also suggest that the unusual gait practised by giraffes may well impair their swimming abilities. Giraffes on land use a synchronous movement of the neck and limbs where backwards and forwards movement of the neck is closely linked to the limb movements. But, with its neck constrained to a sub-horizontal posture where little or no backwards and forwards movement is possible, a swimming giraffe cannot adopt a gait like the one it uses on land.

Quadrupedal mammals – like the horse – typically swim with a trot like that used on land, so it seems reasonable to assume that swimming giraffes should try and swim with a ‘terrestrial’-style gait too. However, note that this argument isn’t completely conclusive, because the swimming gaits of some quadrupedal mammals are very different from their walking or running gaits. Nevertheless, I still think that our assumption is a reasonable one.

Positioned in the water in an uncomfortable pose, afflicted with a relatively high mean density, suffering from substantially high frictional drag, and unable to raise and lower its neck and hence unable to adopt a synchronous gait, we conclude that giraffes would be very poor swimmers, and that it might be assumed that they would avoid this activity if at all possible. Looked at another way, we conclude that giraffes can swim, but not at all well [image below from wikipedia].

Does this have any implications whatsoever for anything?

Making predictions about the floating and swimming abilities of giraffes is fun, but does it have any wider implications? For one thing, it perhaps help make the point that we’re increasingly able to test questions in biology using computational models and simulation, rather than experimentation on real animals. In the case of the question “Can giraffes swim?”, we just aren’t able to use real giraffes, so our approach is – at the moment – the only one we can use to test it.

If giraffes do perform poorly in water – so much so that they avoid crossing large bodies of water should they need do – has this had any impact on their biogeography? If you look at maps of giraffe distribution – and specifically at the ranges of the different giraffe taxa – you’ll see that, in places, rivers and lakes form what appear to be giraffe-proof barriers. The Zambezi and its tributaries might form a southern barrier to Angolan and Thornicroft’s giraffes (Spinage 1968), for example, the Niger and Benue might have prevented the southern spread of the Nigerian giraffe (Hassanin et al. 2007), while the Nile and Great Lakes might have prevented gene flow between Kordofan and Nigerian giraffes (Hassanin et al. 2007) [figure below – from Brown et al. (2007) – shows the approximate ranges and pelage patterns of the extant giraffe taxa].

Unfortunately, we don’t really know enough to be sure whether these distributional limits actually have anything to do with the ability or inability of giraffes to cross water. As we note (Henderson & Naish 2010), big rivers and lakes are formidable barriers to everything terrestrial, not just to clumsy swimmers. Furthermore, the biogeographical history of giraffes extends over many millions of years (and involves Eurasia, too), meaning that climatic changes, changes in drainage patterns, and the terrestrial movement of giraffe populations make it difficult to definitively link any aquatic barriers to giraffe distribution. Others have made similar comments (Cramer & Mazel 2007) [image below: awesome male Masai Giraffe G. camelopardalis tippelskirchi (or G. tippelskirchi), photographed at Lake Manyara National Park, Tanzania. From wikipedia].

Finally, this project is one of several I’ve been involved in that’s gotten a bit of media attention; this attention means that the paper is going to be the subject of more content-free discussion on websites than is usual for peer-reviewed research. Let me assure all concerned readers that this research did not involve the frittering away of pennies otherwise allotted to cancer research or the detection of Earth-killing asteroids. As I’ve said before (and, given that this article is appearing on ScienceBlogs, I’m preaching to the choir anyway), it isn’t widely realised how much work scientists do FOR FREE and FOR FUN, IN THEIR SPARE TIME. Rant over.

Oh, and having mentioned Inside Nature’s Giants, it is my duty to inform you that series 2 starts on June 8th. Episode 1 looks at the anatomy of Carcharodon carcharias, the white shark. This is incredible news; I’ll be writing about the series in due time.

Comments

A few years ago, the excellent BBC series Big Cat Diary featured a scene where a group of giraffes tried to cross the Mara while it was in flood. The giraffes got about half-way across before turning back, and at one stage were in water that submerged them right up to the bases of their necks […] someone told me once that the relevant episode of Big Cat Diary is available online, but I can’t find it today.

I claim no knowledge of quadroped swimming characteristics, but I do know that the position of lungs in humans means that the legs sink. I can’t ague your numbers but it seems that the hind legs should be deeper. I understand that the forelegs are longer, but I feel that the hind legs should be on the bottom longer than the forelegs. I have seen this in some dogs.

Wait, does the leverage of the neck lift the hind legs? Okay, as you were.

Regarding that footage of the wading giraffes… is, by any chance, the depth of the Mara river at the spot where they tried to wade across known? Would it have been deeper than your estimated ‘tipping point’?

Dang, I was just about to ask that question, but Tim beat me to it. Of course, it has an overall similar body structure to a giraffe (longer forelimbs than hindlimbs, big head on the end of a long neck), so maybe it couldn’t swim that well. Of course, the internal structure of a pterosaur is much different than a giraffe, so I’m probably way off. Just guessing here.

Of course, if Quetz couldn’t swim, then it puts a big hole in Henderson’s theory of flightless azdarchids. Azdarchids had a nearly worldwide distribution in the Cretaceous (for example the newly discovered Alanqa in Morocco), and it appears that big azdarchids arose only after the continents split up. If azdarks couldn’t fly, then they would have had to have flown to these distant land masses instead. (Unless there is another way for big animals to get to isolated landmasses. Hitchhike? Yeah, lets go with hitchhike. I can just see a big Quetz trying to get a lift from a trucker, its pteroid bone being used in the traditional hitchhiking gesture.)

“as you’ll know if you watched Inside Nature’s Giants”

I want to, but unfortunately the program hasn’t come on over on this side of the pond.

“If you look at maps of giraffe distribution – and specifically at the ranges of the different giraffe taxa – you’ll see that, in places, rivers and lakes form what appear to be giraffe-proof barriers.”

I wonder what this will mean when the eastern part of Africa breaks off from the rest of the continent. More rapid speciation between both giraffe populations (assuming they survive)?

WRT comment 7…. we should definitely assume that azhdarchids could fly: note that few people in pterosaur-land agree with Don’s suggestion (note that it’s a suggestion; he doesn’t flat out say “Quetz. must have been flightless”). Also, supposing Don is right, his suggestion only applies to the giant forms, not the small- to middling ones. As mentioned here recently, most azhdarchids are in the size range of 2.5-7 m as goes wingspan. As a rough guess, looks like 4-5 m wingspan is ‘most typical’ for the group.

On the depth of the Mara (comment 6): I’m sure this can be found out, but I don’t know the answer. It looks to me like those giraffes are touching bottom at all times in the footage; note that they turn back when the water is lapping around the very base of the neck (hmm.. actually a bit higher than shown in our flotation estimate. I wish I’d had this footage to hand when writing the paper!).

It would be interesting to see how well giraffes breathe when they are in water that deep. We’ve all often heard the reason why sauropods wouldn’t have been semi-aquatic. It would be interesting to see how an extant long necked animal handles the pressure problem.

Also, hooray for the return of Inside Nature’s Giants. Easily one of my favourite nature docs. Besides C.carcharias do we know the line up for the rest of the series?

A large amount of drag and rotational inertia also afflict the model: in fact, it suffered from 13.5% more frictional drag than the horse model

How significant is this, given that the giraffe is bigger and presumably stronger than the horse? Or is it 13.5% more drag per unit mass or something like that?

Also, if there’s such huge uncertainty about the lung volumes of giraffes, I guess there’s also considerable uncertainty about exactly where they lighten the body? Seems like this should have implications for orientation.

But I get the feeling that what you need, to provide a really firm answer, is a giraffe and a swimming pool at least 10′ deep. Many pools are 12′ deep at the deep end and have a very friendly taper as the depth goes from 3′ on down to the deep end. A few safety swimmers, floats, and a line so you can always drag them back to shallower water and you’re all set.

My bet is the giraffe will swim pretty well. That it will puff out it chest and neck a bit and float at a more even keel than the static model predicts.

For swimming in azhdarchids I say they’d be more efficient at it than giraffes; pterosaurs could raise their limbs (otherwise they couldn’t fly), which probably means they’d float more easily. Pterosaur tracts show they swimmed using the backlimbs, so I suppose Quetzalcoatlus in the water would kind of look like a cormorant or a darter, swimming like a frog and floating decently, but not staying above the water like a duck.

Wow. That’s exciting stuff, unfortunatly I am more excited about the return of Inside Nature’s Giants with sharks and lions and omg!
Okies, back on topic. What would happen if a giraffe were to breath in really really deeply and expand their lungs – increase in buyoncy at the front? Considering the uncertainty in lung volume anyhow, what happens when you change that parameter?

Re:
“By scaling up from a horse to a big, male, 1.6 ton giraffe, an enormous projected lung volume of 141 litres was obtained (lung volume is about 6 litres in humans). Projections of this size have long been thought to be erroneous, however (Stahl 1967). A probable lung size about eight times that of a human is generally thought about right… in which case about 48 litres is correct for a 1.1 ton giraffe; this scales to 74 litres for a 1.6 ton giraffe.”

Looked at the other way… I take it the implication is that starting from a giraffe and scaling down would lead to a gross UNDERestimate of a horse’s lung capacity. Are horses outliers, with may more lung capacity than your average’ mammal? … Given their speed and distance running, this wouldn’t surprise me.

Darren and I gave some thoughts on swimming azhdarchids in our 2008 paper after some folk claimed they would swim or dive in search of food. Here’s what we said.

“Although many modern birds regularly swim or dive in pursuit of food, there is no anatomical evidence that azhdarchids did the same [contra. 16,18]. Extant birds demonstrate multiple approaches to feeding on and in water, including surface feeding, plunge diving, and surface diving [101]. Tetrapods that habitually swim possess limbs modified to greater or lesser degrees for propulsion through water, and those that regularly dive bear streamlined bodies to minimise drag [115]. With their relatively small, narrow feet, expansive wings and ventrally oriented skulls atop long, stiffened necks, azhdarchids lack both the propulsion mechanisms and streamlining for efficient movement through water and their limbs show no modifications (e.g. enlarged olecranon or cnemial processes) for swimming. Extant swimming and diving birds also hold their heads close to or above their centre of buoyancy when alighted on the water surface, but the anatomy of azhdarchid cervical vertebrae disallows the possibility of holding the neck at a high angle and may have created issues of stability if the animal were to alight on the water surface. Although other pterosaurs may have been competent swimmers [31], [116], there is no anatomical evidence that azhdarchids were suited for an aquatic existence. Rather, the elongate, slender limbs and proportionally large neck and skull of azhdarchids probably cast them as some of the least aquatically-adapted of all pterosaurs.”

Don and Darren’s research here is corroborates some of these views: gangly azhdarchid limbs (plus membranes) would increase drag no end, and their oversize heads and necks would almost certainly lie along the water surface like the digital giraffe here. I can’t see them being very comfortable in deep water at all, really.

Thanks for comments; the paper has received a fair bit of coverage in the media, most of it good. With reference to the above…

Sauropod swimming (comment 9): everyone agrees that sauropods could swim, probably well. However… Don modelled sauropods for a 2003 paper (it’s cited in the article above), and concluded that their buoyancy (due to their high pneumaticity) would have made them high and unstable in the water and potentially prone to capsizing. I know that some people disagree with this (they counter that the dense-boned limbs might have kept the animals more stable).

In comment 12, Andreas asks how significant “13.5% more drag per unit mass” is. This higher drag is indeed significant, giving the giraffe a Reynolds number twice that of a horse, and meaning that increased effort would be required to swim. The larger size of the giraffe would be disadvantageous here (as would the very long, gracile limbs), as bigger animals have slower muscle contractions and hence have to work harder to move their limbs in swimming.

As for the uncertainty over lung volume, our value is provisional, for sure (74l for 1.6 ton animal), but we justified our reasoning and this volume should be taken as pretty close to real. Given that Don’s other models of extant animals do a good job of replicating the floating postures of the real things (crocodilians, horses, elephants), I would assume that the lung position in the model giraffe, and the resultant posture of the model in the water, are as accurate as they can be for now.

I didn’t, really. The wording of the blog post made it seem like the giraffe has 13.5% more absolute drag, which seemed pretty low, so I asked how significant it was. I also asked if a relative drag, eg. per unit mass, was perhaps meant, as that would make the 13.5% figure more obviously significant.

The excellent Tiger Territory website has a section on how tigers fare against other predatory species like lions, bears, crocodiles and pythons. Generally, it appears that while size matters, the lion is more likely to win the fight.

I know that there are anecdotal reports of lion vs. tiger fights, but all too often such accounts leave much to be desired regarding important details (e.g., the actual body sizes of the respective combatants). I would prefer a more scientific approach for settling this question. In other words, I think Don and Darren should perform a quantitative analysis of the results of 3-D computer model lions & tigers duking it out…

KASELL [announcer]: This giraffe can eat leaves off the limb, but in water my chances look dim. Yet math is clairvoyant and shows that I’m buoyant, they’ve proven that I just might…

Ms. PETERSON [contestant]: Swim.

SAGAL: Swim, yes.

(Soundbite of bell)

SAGAL: Swim, everybody has done it. You’re staring at a giraffe loping it’s graceful way over the savannah, and you say to yourself, I wonder if that bad boy can swim.

(Soundbite of laughter)

SAGAL: Now we know, and no giraffe’s were moistened in making this discovery, using a digital model of a giraffe, submerged in digital water. The authors of a study recently published in the journal, “Theoretical Biology”…

(Soundbite of laughter)

SAGAL: There is such a journal. Found that giraffes do become buoyant in deep water and would be hypothetically able to paddle themselves to shore. You might wonder who might fund such a study, turns out it was a pack of hyenas.

(Soundbite of laughter)

Ms. POUNDSTONE: You know, that’s not what I would ask myself looking at a giraffe.

SAGAL: Really? Can it swim?

Ms. POUNDSTONE: I would want to know like if it could cook or drive.

(Soundbite of laughter)

Ms. POUNDSTONE: If I could animate it in these situations, you know, to really know for sure.

Mr. LUKE BURBANK (Host, “Too Beautiful to Live”): Is it a good tipper?

Thanks indeed John – saves me from listening to the clip myself. I did a radio interview yesterday and the theme there was also humour-based. I don’t mind: Don and I knew all along that people would mostly regard this as a “Why the hell did you bother” bit of research. But – – who’s laughing now? Ha ha.

Inspiration strikes at the strangest of times. After skimming this page earlier today I went shopping, and on my way to the bus I came up with a song. Let’s see if this board can handle the formatting.

The tune is “Running Bear”.

On the banks of the river
stood a hunky bull giraffe.
On the other side of the river
stood his lovely better half.
But alas! He could not reach her.
Neither could she get to him
ever since some meddling teacher
had found out they could not swim!

You can lead giraffes to water
but you can not make them float.
Doctor Naish! We think you ought’er
build those poor giraffes a boat!

Once again, your essays are incredibly thought-provoking and informative. Two thoughts, Camelids share a similar gate with the Giraffids and are fairly capable swimmers. Their buoyancy issues are by no means identical but I am curious if the Llama or Dromedary may have been included in your models?

Secondly, I have heard anecdotal reports that Okapi have been known to escape capture from Ituri hunting parties by plunging into rivers, though this is something they would do only at last resort.

If this is correct, could it be theoretically possible that Okapi represent populations of proto-giraffes that could simply not afford to become clumsy or extremely inefficient swimmers?

Their bodies are approximately horizontal between the water and their necks are held normally, if not maybe a little stretched out.

I see no reason why a giraffe can’t float. With their lungs and rumens there’s plenty of air space within that body cavity. I think that it’ll feel really clumsy in the water, though, and for a prey animal that would be incentive enough not to go diving into rivers and looking like a fool just asking to be eaten.

well good lord, the big question i have here is why on earth are we spending this much time trying to figure this out.. just go get a dang giraffe and drop it in a lake.. it will sink or swim. simple. [from Darren: spam url deleted]